This invention relates to a coaxial engine starter and, more particularly, to a coaxial engine starter in which a solenoid switch, an electric motor and a pinion for driving an engine are axially aligned.
According to the conventional coaxial engine starter 10 shown in FIG. 1, a d.c. electric motor 1 has a hollow armature rotary shaft 2 and a plunger rod 4 of a solenoid switch 3 positioned at the rear end of the d.c. motor 1 is inserted into an inner passage 2a of the armature rotary shaft 2 so that the inserted front end of the plunger rod 4 abuts against the rear end of an output rotary shaft 5 coaxially disposed at the front end of the armature rotary shaft 2 and inserted into the inner passage 2a of the armature rotary shaft 2, whereby the output rotary shaft 5 can be pushed forward.
It is seen that a sun gear 10a is formed on the outer circumference of the front end of the armature rotary shaft 2 and a plurality of planetary gears 10b are in mesh with the sun gear 10a. These planetary gears 10b also mesh with an inner gear 9 formed in the inner circumferential surface of a front frame 13 and are rotatably supported by shafts 11 secured on a carrier 8. The sun gear 10a, the planetary gears 10b, the inner gear 9, the shafts 11 and the carrier 8 together constitute a planetary speed reduction gear 10 which reduces the rotational speed of the armature rotary shaft 2.
On the output rotary shaft 5, an over-running clutch 8 is fitted, of which a clutch inner member 8b is engaged with the output rotary shaft 5 by helical splines 5c of spline formation portion 5b having an outer diameter larger than the inner diameter of the inner passage 2a of the armature rotary shaft 2, so that the output rotary shaft 5 is allowed to axially slide while being rotated by the clutch inner member 8b. On the front end of the output rotary shaft 5, a pinion 6 which engages and disengages an engine ring gear (not shown) is mounted. When the output rotary shaft 5 is moved forward, the pinion 6 meshes with the engine ring gear to rotate the engine.
However, in the coaxial engine starter of the above construction, the front bracket 13 through which the starter is mounted to the engine is made of a metallic material such as die-cast aluminum alloy and has formed therein an inner gear 9 of the planetary speed reduction gear 10. Therefore, in order to constrict precision gear teeth to the inner gear 9, precise machining must be achieved to the front bracket 13. Also, another machining must be carried out to form a receptacle 14 for receiving and supporting a bearing 15 in the front bracket 13 as well as a stepped or a spigot joint between the front bracket 13 and the motor yoke 1. Further, the above-mentioned machining must be achieved for different front brackets of various design for different models or types of the engines to which the starter is to be mounted. Therefore, the number of manufacturing steps and the number of the kinds of parts are large.
Moreover, the front bracket 13, which supports the severe impacts on the bearing 15 upon the engagement of the pinion 6 into the engine ring gear and the hitting of the pinion 6 against the rotating engine ring gear, is entirely made of rigid metal which has substantially no shock-absorbing function. Therefore, the engine ring gear may be damaged or destroyed by the above mechanical impacts. Also, since the front bracket 13 is made of metal, the overall weight of the starter is relatively large, and the inner gear 9 formed in the front bracket 13 makes high metallic noise which degrades value as a commercial product.